1. Field of the Invention
This invention generally relates to a power conserving arrangement for, and a method of, minimizing battery power consumption during stand-by operation of a portable, battery-operated, mobile station and, more particularly, a cellular telephone in a cellular telephone system.
2. Description of Related Art
A typical cellular telephone system includes a plurality of base stations or towers, each serving a pre-assigned geographical cell or region. Each base station transmits messages to a multitude of mobile stations, e.g. cellular telephones, in its region. Each telephone includes a transceiver and a decoder under microprocessor control.
During a stand-by mode of operation, each telephone waits to receive a telephone call. The message transmitted by a respective base station may be a so-called "global" message intended for all telephones, or, most frequently, an individual message intended for just one specific telephone. Hence, the individual message contains a unique mobile identification number (MIN), i.e. the telephone number Each telephone has its unique MIN pre-stored in an on-board memory.
Many messages are transmitted by a respective base station and, of all those many messages, only a very small amount, if any, are intended for a particular telephone. Nevertheless, each telephone, during the stand-by mode of operation, continuously receives and decodes all messages transmitted by the respective base station until the decoder of a particular telephone recognizes its MIN, after which, the telephone operates in a talk (call in progress) mode. The telephone transmits and receives data, including voice data, to and from the base station in the talk mode.
It will be seen that conventional cellular telephones in current use consume electrical power in both the talk and the stand-by modes. In current portable battery-operated telephones, the on-board battery typically has a working lifetime of approximately 8 hours in the stand-by mode, and about 1-2 hours in the talk mode. The battery must then be re-charged or replaced to continue telephone service. A major electrical current consumer on-board the battery-operated cellular telephone during the stand-by mode is the receiver section of the transceiver which, as previously described, is continuously on while the telephone is waiting to decode its MIN. The microprocessor and other electronic components on-board the telephone are also energized during the stand-by mode and additionally contribute to current drain on the battery. The need to increase the battery working lifetime between re-charges and/or battery replacement is self-evident.
To aid in understanding the invention described herein, a brief review of the prior art structure of the message transmitted by the base station during stand-by operation is presented. The message is a digital stream of bits, and may have one or more words. Usually, a message includes two words. FIG. 1 schematically shows the prior art structure of each word of the message. Each word contains forty bits. The first twenty-eight bits are message data containing, among other things, the MIN and/or a global message and/or a channel assignment message, etc. The last twelve bits are a sequence or parity field of check bits (BCH) and is a block code parity check sum. The BCH parity field confirms that the message data in the first twenty-eight bits were correctly received.
To overcome the problem of messages that are sometimes lost by rapidly changing radio signals, each word of the message is transmitted from the base station to each portable telephone five times. For a message to be validated, each word must be correctly received at least three out of the five times before the telephone will respond to the message. In addition, to compensate for burst errors, words are interleaved and transmitted in a format based on whether the MIN is odd or even.
FIG. 2 schematically shows the prior art structure of the interleaved format wherein each word A (designated for even telephone numbers) and each word B (designated for odd telephone numbers) is repeated five times and, for each repetition, the even word A is alternated with the odd word B. In addition, FIG. 2 shows a dotting sequence D which is a sequence of ten bits that advises the telephone that a synchronization word S is coming. The dotting sequence produces a 5 kHz frequency signal which is a precursor and a gross indicator that a message is about to start. The synchronization word is a sequence of eleven bits, and includes a synchronization pattern by which an internal clock of the telephone is synchronized to the base station transmitter.
Also imposed on the message data stream are busy-idle bits which are schematically shown in FIG. 3. A busy-idle bit is sent every ten bits of the message to indicate the status of the system channel. If the busy-idle bit is set to logic 1, then the channel is not busy. If the busy-idle bit is set to logic 0, then the channel is busy. The data rate for transmitted bits is 10 kbps. Hence, as shown in FIG. 2, 463 bits are transmitted in 46.3 msec, and is the total time in which one odd and one even word is transmitted five times in an interleaved format.
As previously noted, a message may, and typically does, contain more than one word. When this happens, each word also advises the on-board microprocessor that more words for the complete message are coming.
FIG. 4 schematically shows the prior art structure and duration of a complete message that consists of two words wherein word C is the second word of the message for an even telephone number which had word A as the first word, and wherein word D is the second word of the message for an odd telephone number which had word B as the first word. A two-word message takes 92.6 msec to be completely transmitted.